Description
Important: This diagnostic procedure should only be used to diagnose a suspected inaccurate CNG fuel level reading and not a malfunctioning fuel gauge.
The fuel gauge receives a signal from the engine control unit (ECU) in order to display the CNG fuel level. The fuel pressure sensor (FPS) and the fuel temperature sensor (FTS) each output a voltage to the ECU. Using these voltages, the ECU calculates CNG fuel level and outputs a pulse width modulated (PWM) signal to the fuel gauge.
The ECU is unable to run a diagnostic to detect a malfunctioning FTS. A faulty sensor could cause an inaccurate fuel level signal to be output to the fuel gauge.
Preliminary Checks
Inspect for the following conditions:
| • | Verify that the CNG fuel tank is at least 1/3 full. Add fuel if needed. If fuel is added, record the fill pressure at the refueling station. |
| Important: If the tank is fast-filled, the fuel will be heated as it is compressed into the tank. This may affect any fuel temperature readings taken for up to three hours after filling. As the fuel cools, the tank pressure may drop as much as 400 psi depending on the quantity of fuel added. |
| • | Verify that no AF Diagnostic Trouble Codes (DTCs) are set. |
| • | Verify that the fuel pressure sensor (FPS) electrical connector is connected. |
| • | Verify that the fuel temperature sensor (FTS) electrical connector is connected. |
Note that the fuel gauge will display the dormant fuel level for ten seconds when the fuel gauge selector switch is pressed. The gauge will then return to normal operation.
Fuel Level Output Check
- Measure the FTS voltage output. Convert the voltage to a temperature and compare the reading to the IAT sensor. Refer to Fuel Temperature Sensor Output Voltage Versus Temperature Resistance .
- Measure the FPS voltage output. Convert the voltage to a pressure and compare it to the fill pressure reading. Refer to Fuel Temperature Sensor Output Voltage Versus Pressure .
- Determine the percent full of the CNG fuel tank using the FPS and FTS readings. Refer to CNG Temperature Versus Percent Full and PWM Output . The percent full should match the fuel gauge reading. For example, with a reading of 0%, the fuel gauge should read E. With a reading of 100%, the fuel gauge should read F.
Important: Removal of the air filter element with the hood open will result in a more accurate ambient temperature reading. The FTS reading may be higher than the ambient temperature if the tank has recently been filled.
Important: If the tank has just been fast-filled, the tank pressure will drop as the fuel cools. This pressure drop may be as much as 400 psi depending on the quantity of fuel added.
